For a power amplifier provided in an output part of the transmitter of a radio communication system, the compatibility of a low distortion and a high efficiency is required. The power amplifier has a classification that a transistor is used as a current source or a switch. The amplifier using the transistor as the current source includes an class A amplifier, an class AB amplifier, a class B amplifier and a class C amplifier. Further, the amplifier using the transistor as the switch includes a class D amplifier, an class E amplifier and an class F amplifier.
As the high frequency power amplifier for amplifying a modulation signal including an envelope variable component, an A class linear amplifier or an class AB linear amplifier has been used to linearly amplify the envelope variable component. However, the power efficiency of the linear amplifier has been disadvantageously lower than that of a class C or an class E non-linear amplifier. Accordingly, when the linear amplifier is used in a portable radio device such as a portable telephone, a portable information terminal, or the like having a battery as a power source, a using time has been inconveniently decreased. Further, in a base station device of a mobile telecommunication system in which a plurality of transmitters using a large quantity of power is installed, the device is undesirably enlarged and the quantity of generated heat is inconveniently increased.
Thus, as a transmitter having a highly efficient transmitting function, an EE & R (Envelope Elimination and Restoration) transmitter is proposed which includes an amplitude phase extracting part, an amplitude modulating part, a phase modulating part and a non-linear amplifying part, inputs a signal of a prescribed envelope level to the non-linear amplifying part and uses the non-linear amplifier having a good efficiency as a high frequency amplifier. Further, a transmitter has been also known in which the non-linearity of an envelope signal of a non-linear amplifier is compensated by a negative feedback to suppress an amplitude distortion.
FIG. 9 is a block diagram showing the structure of the above-described EE & R transmitter as a first existing example. The transmitter of the first existing example includes a transmit data input terminal 111, an amplitude phase extracting part 112, an amplitude modulating part 113, a phase modulating part 114, a non-linear amplifying part 115 and a transmit output terminal 116.
In FIG. 9, assuming that a transmit data signal Si(t) inputted from the transmit data input terminal 111 is expressed bySi(t)=a(t)exp[jø(t)]  (1),amplitude data a(t) and phase data exp[jø(t)] are extracted from the Si(t) by the amplitude phase extracting part 112. The source voltage value of the non-linear amplifying part 115 is set by the amplitude modulating part 113 on the basis of the amplitude data a(t). On the other hand, a signal is generated by modulating carrier wave angular frequency ωc by the phase data exp[jø(t)]in the phase modulating part 114 to be Sc and Sc is inputted to the non-linear amplifying part 115.Sc=exp[ωct+ø(t)]  (2)
A signal is generated by multiplying the source voltage value a(t) of the non-linear amplifying part 115 by the output signal of the phase modulating part 114. An RF signal Srf obtained by amplifying the obtained signal by a gain G of the non-linear amplifying part 115 is outputted to the output of the non-linear amplifying part 115.Srf=Ga(t)Sc=Ga(t)exp[ωct+ø(t)]  (3)
As described above, since the signal inputted to the non-linear amplifying part 115 is the signal of a prescribed envelope level, a non-linear amplifier having a good efficiency as a high frequency amplifier can be used. Thus, the transmitter with a high efficiency can be realized.
In the first existing example, the detail of the amplitude modulating part 113 is not illustrated. However, the amplitude modulating part 113 uses a structure that includes, for instance, a DA (digital-analog) converting part, a pulse width modulating part, a switch and a low-pass filter which are connected in order and in series to input source voltage to the switch. In the amplitude modulating part 113, the amplitude data as a digital value is converted to an analog signal in the DA converting part and the pulse width of the analog signal is modulated in the pulse width modulating part. The switch is switched in accordance with the pulse output of the pulse width modulating part. The output of the switch is smoothed in the low pass filter to be an amplitude modulating signal and is applied as the source voltage of the non-linear amplifying part 115 (for instance, see Non-Patent Document 1).
Further, the phase modulating part 114 employs a structure using a PLL (Phase-Locked Loop). That is, the PLL, whose detail is not illustrated, is provided in which for instance, a phase frequency comparing part, a low-pass filter and a voltage control oscillator are connected in order and in series and a part of the output of the voltage control oscillator is fed back as a feedback signal to the phase frequency comparing part through a frequency divider. Further, an output of a ΔΣ (delta sigma) modulating part is inputted to the above-described frequency divider. In the phase modulating part 114, the frequency of a signal obtained by dividing the frequency of the output of the voltage control oscillator by the frequency divider is compared with a reference frequency in the phase frequency comparing part to output a difference between both the frequencies. The output of the phase frequency comparing part passes through the low-pass filter to become the control voltage of the voltage control oscillator and the output of the voltage control oscillator is locked by a prescribed phase and frequency. In the above-described PLL, the frequency dividing ratio of the frequency divider is changed in accordance with a signal obtained by performing a delta sigma modulation to phase data so that a phase modulation can be applied to the output of the voltage control oscillator (For instance, see Non-Patent Document 2).
FIG. 10 is a block diagram showing the structure of a transmitter having a negative feedback as a second existing or usual example. The transmitter of the second usual example includes a transmit data input terminal 111, an amplitude phase extracting part 112, an amplitude modulating part 113, a phase modulating part 114, a non-linear amplifying part 115, a transmit output terminal 116, a directional coupling part 117, an envelope detecting part 118, an AD (analog digital) converting part 119, an adding part 120 and an amplifying part 121. The same components as those of the transmitter shown in FIG. 9 are designated by the same reference numerals.
Now, an operation of the transmitter of the second existing or usual example will be described below. The transmitter of the second usual example feeds back the envelope component of an RF signal as an output of the non-linear amplifying part 115 in addition to the same operation as that of the transmitter as the first usual example shown in FIG. 9. The output of the non-linear amplifying part 115 is allowed to branch by the directional coupling part 117 and inputted to the envelope detecting part 118 to detect the envelope signal of the RF signal. The detected envelope signal is subjected to an analog digital conversion in the AD converting part 119 and the analog digital converted envelope signal is subtracted from the original amplitude data in the adding part 120, then amplified in the amplifying part 121 and inputted to the amplitude modulating part 113. The non-linearity of the envelope signal of the non-linear amplifying part 115 is compensated by the above-described feedback so that an amplitude distortion can be suppressed (For instance, see Non-Patent Document 3).    (Non-Patent Document 1) Peter B. Keningstopm, “HIGH-LINEARITY RF AMPLIFIER DESIGN” first edition, ARTECH HOUSE, INC., 2000, p 426–443    (Non-Patent Document 2) R. A. Meyers and P. H. Waters, “Synthesizer review for PAN-European digital cellular radio” poc. IEE Colloquium on VLSI Implementations for 2nd Generation Digital Cordless and Mobile Telecommunications Systems, 1990, p. 8/1–8/8    (Non-Patent Document 3) Peter B. Kenington, “HIGH-LINEARITY RF AMPLIFIER DESIGN” first edition, ARTECH HOUSE, INC., 2000, p. 156–161
However, in the transmitter of the first usual example shown in FIG. 9, since an amplitude signal and a phase signal reach the non-linear amplifying part 115 through different paths, an output signal is undesirably distorted due to the difference of delay time between the signal path of an amplitude modulation and the signal path of a phase modulation.
The transmitter of the second usual example shown in FIG. 10 has a structure for reducing an amplitude distortion by a negative feedback loop. To more increase a quantity of decrease of the amplitude distortion, a loop gain needs to be increased. Accordingly, the stability of the negative feedback loop is disadvantageously deteriorated.
The present invention is devised to solve the above-described problems and it is an object of the present invention to provide a transmitter in which power efficiency is good and a stable signal having little distortion can be outputted.